An eyepiece optical system configured of a negative lens element, a positive lens element and a positive lens element (three lens elements), in that order from the object side to the eye side (Patent Literature 1), an eyepiece optical system configured of a positive lens element, a negative lens element and a positive lens element (three lens elements), in that order from the object side to the eye side (Patent Literature 2 and 3), and an eyepiece optical system configured of a negative lens element, positive lens element, a negative lens element and a positive lens element (four lens elements), in that order from the object side to the eye side (Patent Literature 4) are known in art as examples of an eyepiece optical system for magnified viewing of an object that is displayed on a miniature image display device such as a screen, a focal plane plate, an LCD or an organic EL device, etc.
In such an eyepiece optical system, a view is demanded in which the apparent visual angle is enlarged by magnifying the object image to a certain extent. In particular, in the case of the image display device, which displays an object image on an electronic viewfinder, having a horizontal rectangular shape with an aspect ratio of 16:9, etc., since the size of the image display device appears smaller than for an image display device having an aspect ratio of 4:3 at the same magnification, a higher magnification ratio is demanded. Hence, it is conceivable to increase the apparent visual angle by reducing the focal length of the eyepiece optical system. Furthermore, in order to achieve a miniaturized electronic viewfinder, a miniaturized image display device can be utilized, thereby reducing the focal length of the eyepiece optical system. Whereas, since an eyepiece optical system is directly viewed by the user's eye, if the distance to the eyepoint or the pupil diameter is reduced in proportion to the focal length, shading can easily occur with a slight shift in position of the user's eye and aberrations increase, causing difficulties in viewing.
The eyepiece optical system of Patent Literature 1 teaches a lens arrangement configured of a negative lens element, a positive lens element and a positive lens element (i.e., three lens elements); however, the apparent visual angle is only 25 degrees (which is small), and since the pupil diameter is small, if the position of the user's eye shifts in a direction orthogonal to the optical axis, shading or deterioration in the optical quality easily occurs.
The eyepiece optical system of Patent Literature 2 teaches a triplet lens arrangement provided with a positive lens element, a negative lens element and a positive lens element, and although the apparent visual angle is wider than that of Patent Literature 1, since the refractive power of the middle negative lens element is too weak, correction of aberrations is insufficient and the pupil diameter is small.
The eyepiece optical system of Patent Literature 3 also teaches a triplet lens arrangement provided with a positive lens element, a negative lens element and a positive lens element, and although the refractive power of the middle negative lens element is strengthened, the apparent visual angle is only 28 degrees (which is small). Furthermore, in order to increase the apparent visual angle while maintaining the aberration quality and the eyepoint position, it is necessary to increase the refractive power of each lens element, so that the aberration correction is insufficient, thereby deteriorating optical quality.
The eyepiece optical system of Patent Literature 4 teaches a lens arrangement configured of a negative lens element, a positive lens element, a negative lens element and a positive lens element (i.e., four lens elements); however, since a large image display device having a diagonal size of approximately 90 mm is utilized, enlargement of the apparatus including the eyepiece optical system is unavoidable. Furthermore, since the profile and refractive power of the negative lens element that is provided closest to the object side is inappropriate, telecentricity is deteriorated and the apparent visual angle cannot be widened without using a large image display device.